A variety of therapies, such as neurostimulation or therapeutic agents, e.g., drugs, may be delivered to a patient to treat chronic or episodic pain. Examples of neurostimulation therapies used to treat pain are transcutaneous electrical nerve stimulation (TENS), percutaneous electrical nerve stimulation (PENS), peripheral nerve stimulation (PNS), spinal cord stimulation (SCS), deep brain stimulation (DBS) and cortical stimulation (CS). Examples of drugs used to treat pain are opioids, cannabinoids, local anesthetics, baclofen, adenosine and alpha-blockers.
PNS, SCS, DBS and CS are typically delivered by an implantable medical device (IMD). An IMD delivers neurostimulation therapy via electrodes, which are typically coupled to the IMD by one or more leads. The number and positions of the leads and electrodes is largely dependent on the type or cause of the pain, and the type of neurostimulation delivered to treat the pain. In general, an IMD delivers neurostimulation therapy in the form of electrical pulses.
SCS involves stimulating the spinal cord at specifically targeted locations, typically via leads and electrodes that are either surgically implanted post laminectomy, or inserted percutaneously in the epidural space. Delivering stimulation to the appropriate location on the spinal cord causes paresthesia that overlay the pain region to reduce the area of perceived pain. SCS can result in the patient experiencing paresthesia in a relatively large area, including more than one limb.
SCS has been shown to be effective for axial or longitudinal back pain, failed back surgery syndrome (FBBS), cervical pain, C1-C2 cervicogenic headaches, supra-orbital pain, facial pain, inguinal and pelvic pain, and chest and intercostal pain. As examples, electrodes for SCS may be implanted in the epidural space near vertebral levels T8-T10 to treat axial back pain, over the dorsal columns at vertebral levels T10-L1 to treat pain in the back, legs, ankles or feet, or over the dorsal roots, i.e., at the dorsal root entry zone, of vertebral levels L3-S1. SCS may be most effective for neuropathic pain, such as neuropathy or radiculopathy that involves a significant portion of one limb and more than one dermatome.
PNS is typically used to treat patients suffering from intractable pain associated with a single nerve. PNS places a group of electrodes in very close proximity to, e.g., in contact with, and approximately parallel to a major nerve in the subcutaneous tissue. PNS may also place a group of electrodes in very close proximity to a nerve that may be deeper in the limb. Placing electrodes in very close proximity to the nerve may ensure that only fibers within that nerve are activated at low amplitudes.
PNS electrodes may be located on percutaneous leads, but for stability and to prevent stimulation of other tissues proximate to the target peripheral nerve, PNS electrodes are generally located within insulative material that wraps around a nerve, i.e. cuff electrodes, or on one surface of a flat paddle of insulative material placed under a nerve. In any case, the electrodes for PNS are placed in close proximity to the nerve “upstream” from the source of damage or pain, e.g., closer to the spinal cord than the region of damage or pain. When electrodes are implanted upstream, the paresthesia resulting from PNS may extend to a broader area innervated by the target peripheral nerve. The most common upper extremity nerves treated with PNS are the ulnar nerve, median nerve, radial nerve, tibial nerve and common peroneal nerve.
DBS and CS can be used to treat neuropathic and nociceptive pain through delivery of stimulation to various structures of the brain. DBS may treat pain through delivery of stimulation to gray matter within the midbrain, or the thalamus, via electrodes implanted in the brain. CS may treat pain through delivery of stimulation to the sensory and/or motor cortex via electrodes placed in or on the cortex.
Therapeutic agents that treat pain may be delivered by an implantable pump, external pump, transdermally, or orally. Typically, an implantable pump delivers one or more therapeutic agents to a target location via a catheter. The target location may be intrathecal or extradural.
The pain experienced by a patient may be complex and/or multifocal. Complex or multifocal pain may include pain experienced by a patient at different locations of the body, pain attributable to different causes or pathologies, and/or pain of different types, e.g., neuropathic and/or nociceptive pain. For some patients with complex and/or multifocal pain, any one of the pain treatment modalities identified above may be unable to completely treat the experienced pain. For example, SCS may not adequately treat pain in a large number of cases, perhaps the majority, because it has been shown to help neuropathic, but not nociceptive, pain states. Nociceptive pains can come from pressure, inflammation, and temperature changes.
Further, over time, the nervous system of a patient may accommodate to a particular treatment modality. Such neural accommodation may render a previously effective modality, or dose or intensity for the modality, ineffective. Neural accommodation may result from noxious sensations being rerouted to traverse alternative pathways in the nervous system that are not affected by the accommodated modality, at least at the current dose or intensity. Simply increasing the dose or intensity of a current modality to overcome accommodation may not be effective, or may be undesirable for a variety of reasons, such as increased battery or reservoir consumption, increased side-effects, or increased likelihood of chemical dependency.